Tin (II) (stannous) ions, provided in oral compositions by stannous chloride and/or other stannous salts, have long been valued for the multiple benefits that they can afford, including antimicrobial effects, control of breath malodor, control of dental plaque growth and metabolism, reduced gingivitis, decreased progression to periodontal disease, reductions in dentinal hypersensitivity, and reduced coronal and root dental caries and erosion.
The use of stannous chloride as a raw material can pose problems. Stannous (II) chloride, both dihydrate and anhydrous forms, are hygroscopic materials and thus attract water vapour from the air through both absorption and adsorption. This makes the powdered compound sticky. Particles can bind together, agglomerates can form during transit and storage and thus flowability during processing is difficult.
It has been a problem in the art to ship, store and handle stannous (II) ion salts due to the handling problems discussed above. Accomplishing flow of stannous ion salts from storage bins has proven to be difficult. Ideal flow design would be a simple storage bin with wall angles steep enough to promote mass flow. However, stannous chloride cakes so readily that simple mass flow design does not work. Both the Peschl Shear Tester and bin design calculations indicate that the wall angles of the bin need to be near vertical with a large outlet dimension.
The use of an anti-caking agent can improve the flow, decrease the compaction and therefore decrease restricted flow during processing. Anti-caking agents function either by adsorbing excess moisture or by coating particles to make them less prone to water adsorption. Other compounds are known to experience similar problems to stannous chloride, for example, potassium nitrate. WO 2007/136381 discloses that the addition of dipotassium hydrogen phosphate and potassium dihydrogen phosphate to potassium nitrate maintains a free flowing state.
The production of an abrasive toothpaste containing stannous has also presented compatibility problems. Although silica abrasives are widely used within toothpastes, the surface hydroxyl groups on the silica particles can deactivate stannous ions in combination with other factors, e.g. pH and water. The combination of silica with stannous ions has shown loss of bioavailability of the stannous over the shelf life of the toothpaste. The present invention discloses a composition comprising stannous chloride with silica as an anti-caking agent. For simplicity, the use of dentifrice grade silica as an anti-caking agent to improve the flow is advantageous for both manufacturing simplicity and well accepted worldwide safety and regulation profiles. It has been found that stannous chloride dihydrate can be maintained in a free flowing condition when admixed with silica dioxide with minimal loss in the activity of the stannous chloride due to the small amounts of silica utilised and the low water activity which inhibits stannous ion transfer to the silica.